This invention generally relates to optical receivers and it particularly pertains to the technology of optical receivers for use in optical remote control receivers operable to perform conversion of optical signals transmitted from the optical remote controller into electrical signals.
A typical optical remote control receiver is made up of an optical receiver, such as a photodiode (PD), operable to convert an optical signal into an electric current, and a peripheral circuit operable to convert a photocurrent produced in the optical receiver when light falls thereupon into a signal distinguishable by a logic circuit such as microcomputer.
Generally speaking, as the distance between the optical receiver and the transmitter, i.e., the receiving distance, increases, the optical receiver receives weaker light. Stated another way, as the photodetection sensitivity of an optical remote control receiver increases, its receiving range likewise increases. Conversely, as the photodetection sensitivity decreases, the receiving range likewise decreases. The long receiving range provides some advantages. For example, one advantage is that the transmitter may reduce its optical signal output for the electric battery to last for a longer time. Another advantage is that improvements in handling can be made.
Accordingly, there have been strong demands in the market for optical remote control receivers having a receiving range of 10 meters or more. Demand for high-performance optical receivers superior in photodetection sensitivity is therefore high.
Optical remote control receivers generally have domestic uses, which means that their photodetection sensitivity is greatly affected by the existence of electromagnetic noise from, for example, fluorescent lamps. In other words, the optical remote control receiver receives electromagnetic noise in addition to optical signals transmitted from a transmitter. The received optical signals are lost in the noise, leading to a drop of the photodetection sensitivity.
With a view to getting rid of such unwanted electromagnetic noise, various approaches have been made to optical receivers.
For example, Japanese Patent Application Laying Open Gazette No. 6-69409 shows a technique that the entirety of an optical receiver is shielded from electromagnetic noise by a metallic shell. Japanese Patent Application Laying Open Gazette No. 6-291356 shows a technique that a light-transmissive conductive film is formed on the surface of a light receiving section. Japanese Patent Application Laying Open Gazette No. 2-275680 shows a technique that the entire light receiving surface of a photodiode is shielded by a diffused region. Japanese Utility Model Application Laying Open Gazette No. 4-40553 shows a technique that only an area of the light receiving surface in the vicinity of a noise source is shielded by a diffused layer.
The above-listed prior art techniques are problematic in the following aspects. The technique of shielding the entirety of an optical receiver from electromagnetic noise is unattractive in the aspect of dimensions as well as in the aspect of costs. The technique of forming a light-transmissive conductive film on the light receiving surface is not attractive, either, for the reason that although the conductive film is a trans illuminous film its transmission rate is not 100%. Accordingly, there exist limits of improving the sensitivity of photodetection. Preferable dimensions, costs, and photodetection sensitivity would be best achieved by the technique of shielding a light receiving surface by a diffused layer.
Even such a technique is problematic. Although the technique may achieve a reduction of the effect of electromagnetic noise, such achievement will not much contribute to providing an improved photodetection sensitivity. It is hard to provide a receiving range in excess of 10 meters. Additionally, it is almost impossible to locate an electromagnetic noise source in general homes, and it is therefore practically impossible to shield only a near-noise source area of the light receiving surface by diffused layer.
Having conducted examinations based on experiments, the inventors of the present invention discovered that there occurs spontaneous noise in optical receivers due to the contact of a light receiving layer and a diffused layer used to shield the light receiving layer. Such spontaneous noise is a new bar to improvement in the optical receiver photodetection sensitivity.